The subject invention concerns the field of OSD systems for a television receiver.
The term xe2x80x9ctelevision receiverxe2x80x9d as used herein includes television receivers having a display device (commonly called xe2x80x9ctelevision setsxe2x80x9d), and television receivers not having a display device, such as VCRs, videodisc players, digital videodisc units (DVD), cable converter boxes, satellite receivers, and the like.
Services such as closed captioning, XDS (Extended Data Service) and Starsight(copyright) transmit data to television receivers within predetermined undisplayed (blanked) television horizontal line intervals. Television receivers include a signal processor or decoder, e.g., a microprocessor, microcomputer, or microcontroller, that extracts the auxiliary information component. An example is a closed caption decoder. The auxiliary information component of a television signal has heretofore consisted largely of text and control characters. It has been proposed that graphics data also be sent via an auxiliary information component of a television signal using data protocols such as those used for closed captioning, XDS and Starsight(copyright) data.
In such a system, a graphical on-screen display (OSD) system in a television receiver receives a graphic image representative signal from a transmitter. The graphic image is partitioned into an array of picture elements (pixels). The pixel array contains a predetermined number of rows, each row containing a predetermined number of pixels. The image representative signal includes sequential data values representing the colors of respective pixels in the array. Each sequential data value points to a location in a color palette. The locations in the color palette, in turn, contain data representing the desired color for the corresponding pixel. The color representative data is processed by an OSD controller in the television receiver to condition the television receiver to display a pixel of the desired color.
For example, the StarSight(copyright) system will have the capability of producing graphic images on the TV screen. These images comprise an array of approximately square pixels where each pixel has a height of 2 TV scan lines. The active area of a typical NTSC TV screen, with 3:4 aspect ratio, can be filled by about 86,400 (240xc3x97360) pixels. Because auxiliary information such as StarSight(copyright) data is transmitted using a relatively low bandwidth protocol similar to closed captioning, the time required to transmit a full screen graphic image would be unacceptably long. Therefore, the maximum size of a graphic image may be limited to less than 5% of the screen area which permits transmission to occur within an acceptable period of time. Recent proposals call for data representing each pixel to be transmitted as a 4 bit binary value specifying 1 of 16 possible colors for that pixel. Each received data value represents the address in a sixteen entry color palette where data representing the specified one of 16 possible colors used in any given graphic image has previously been stored in binary format.
Closed caption text displayed on screens of existing TV receivers, e.g., those manufactured by Thomson Consumer Electronics, Inc., are made up of pixels which are about the same size as those proposed to make up the graphic images in the proposed StarSight system. It is well known that diagonal strokes of characters like A and W in the closed caption text images look somewhat ragged and coarse on the TV screen. To improve the appearance of diagonal character strokes, the textual OSD controller hardware in such television receivers has been designed to smooth these strokes by generating and appropriately placing along the diagonals small, or rounding, pixels of xc2xc the size of normal pixels, i.e., having a height of one scan line and a width one-half the width of the normal pixels. The displayed character images are made up of both normal foreground pixels and small, half-size, rounding foreground pixels of one color constituting the strokes of the character, and xe2x80x9cbackgroundxe2x80x9d pixels of a different color surrounding the foreground pixels. The rounding pixels are always generated in the xe2x80x9cforegroundxe2x80x9d color. This operation may be easily accomplished in current textual on-screen display (OSD) controllers, because they xe2x80x9cknowxe2x80x9d that a character is being drawn, xe2x80x9cknowxe2x80x9d what that character is, and xe2x80x9cknowxe2x80x9d where to add the rounding dots in that character image to make the most pleasing character image. An example of a display system capable of displaying text characters and of displaying graphics using stored graphics xe2x80x9cprimitivesxe2x80x9d is described in EP 0651571. An example of a display system that provides for rounding or smoothing of diagonal edges is described in xe2x80x9cA Single Chip Line 21 Captioning Decoderxe2x80x9d, N. F. Hurley, IEEE Transactions on Consumer Electronics, vol. 38, no. 3, pages 261-267.
However, when a graphical image, which may include image elements representing textual characters as well as other graphic objects, is transmitted to the television receiver, the OSD controller has no such knowledge. The low resolution of the graphics image represented by the transmitted signal does not, by itself, provide the level of detail needed to produce pleasing smoothed, rounded, character, or other edges in the graphic elements. The proposed graphical display, however, would also benefit from application of suitable edge smoothing. For best results, the rules for generating rounding elements are more complex for graphics, because generally more than 2 colors are involved and the concept of xe2x80x9cforegroundxe2x80x9d and xe2x80x9cbackgroundxe2x80x9d may not be relevant. Also, rounding may be undesirable in some situations.
A graphical on-screen display (OSD) system incorporating the invention processes an auxiliary information component of a television signal and includes an edge detector for determining that a textual or other graphical edge is present in a graphical image. The system further includes an edge smoother for determining whether the detected edge is to be smoothed or rounded, or is not. In addition, the system automatically determines the proper location and color for the smoothing pixels without prior knowledge of the image element shape. The smoothing pixels may be any color in the color palette.